The invention relates to a force sensor, and especially to a compressive force sensor, such as a pressure sensor.
It is known in the art to produce conventional compressive force sensors according to a so-called ceramic multilayer technique disclosed in DE 44 41 487.0. The technique involves applying a number of pastes, or "green films", to a substrate, and bonding the pastes to the substrate by a firing process, such as sintering. In order to carry out the above process, the layers are arranged such that they define at least one diaphragm extending over a recess in the sensor. As a result of the above configuration, a force applied to the diaphragm, for example, a pressure force external to the sensor, causes a deflection of the diaphragm which is proportional thereto. In the process disclosed in DE 44 41 487.0, diaphragms such as the one described above may be produced which are both largely stress-free and crack-free.
Furthermore, it is known in the prior art to integrate passive and/or active electronic components into force sensors such as the one described above. These electronic components supply a measuring signal which is proportional to a deflection of the diaphragm in response to a force being applied thereto from the exterior of the sensor, that is, in response to an external force. For the above purpose, piezoresistive resistors may be used which change their resistance value as a result of the deflection of the diaphragm. Alternatively, capacitive devices are known which change their capacitance likewise as a function of the deflection of the diaphragm. The measuring signals resulting from the deflection of the diaphragm are fed to an evaluating circuit, which in turn generates a signal analog to the external force and/or triggers a reaction, as is well known.
A disadvantage of the force sensors according to the prior art is that they are subject to an ageing process during the course of their intended use, which ageing process affects their reliability. The prior art does present measures for subjecting a conventional force sensor, at selectable intervals, to a functional test. According to the above functional test, a check is performed as to whether the force sensor being tested generates a sensor signal in response to an external force being applied thereto. However, one disadvantage of the above is that possible changes in the sensor signal resulting from an ageing of the force sensor are not accounted for.